For instance, I realise being able to absorb simple sugars in the mouth is pivotal in the rapid action of oral glucose gel. Thus I was wondering what nutrients in general can be absorbed directly within the mouth, and at what speed?

This could be a fairly long list, though off the top of my head Vitamin C and Glucose would be good places to start for nutrients and drugs GTN and Aspirin. This Question is also related.
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Rory M♦Jun 22 '13 at 13:47

4 Answers
4

Quite a bit can be absorbed through the mouth. Most commonly, starches are broken down to maltose (two glucose molecules formed by a condensation reaction) and are easily absorbed by the bloodstream.

A lot of other factors balance into this, ie pH, lipid solubility, and molecular weight. Generally, if a substance is easily dissolved in saliva, it can be administered buccally or sublingually because the only remaining step is the diffusion into the subepithelial capillaries. In regards to speed, anything properly absorbed via buccal or sublingual administration goes to work much faster than a standard oral medication, and with a higher availability in the bloodstream. You are taking first pass metabolism and enzyme breakdown in the stomach out of the equation by diffusing the drug directly into the bloodstream.

There is quite a laundry list of medications that can be administered transbucally or sublingually, just a few of these:
Nitroglycerin
Acetylsalicylic acid (aspirin)
Glucose gel (as you mentioned)
Fentanyl (a narcotic painkiller)
buprenorphine (for opioid dependency)
Benzodiazepines (alprazolam, clonazepam)

so you said pH and lipid solubility... if the salivary secretions are alkaline would that mean that alkaline compounds would be deprotonated and more lipophilic, hence better absorbed?
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Jasand PruskiJan 27 at 1:41

Actually, absorption does take place through the mouth.There is a ptyalin enzyme in the saliva which hydrolyzes carbohydrates of the food. These contents are then absorbed in the blood through the facial vein.The facial vein opens into subclavian vein,and it opens into the superior vena cava.

The mouth or the oral cavity is lined by oral mucous membrane. The oral mucous membrane has two parts the epithelium and its supporting connective tissue known as lamina propriya. The charecter of the epithelium and lamina propriya varies to a great extent from the gingiva and hard palate to the floor of the mouth based on functional requirements. However basically the mucous membrane lining the oral cavity is not designed for absorbing nutrients, which was the primary question.
As for the oral glucose gel - read the American Red Cross, Scientific Advisory Council recommendation that the buccal absorption of glucose is limited and not recommended. ARC SAC Advisory